Well, maybe there were some clues in the second installment that may open up a few doors for the 3rd movies antagonist.
1) Two-Face will be back. My guess is that Dent, having no place in the legal system starts brandishing out his own "justice" against the people who he perceived were responsible for Dawes death, Batman and Gordan, though indirectly at first. Sort of like the Joker, only more methodical.
2) Batman/Wayne need a new love interest. And a potential hint thrown by Fox, Batman's suit doesn't provide much protection against cats. Perhaps there's a dual role, one as Waynes new love interest who happens to be 'Catwoman' as well. Obviously the whole cat thing will have to be toned WAY down. Say she's a cat-burglar that maybe the media give a pet name to "The Cat Woman" hired by Harvey/underworld figures, to steal from Wayne and along the way sort of get tangled between Wayne/Batman. Or something along those lines. Two face being the main villian, "Cat Woman" being sort of the secondary villain. Again, the whole cat thing would need to be far darker, and way less about being a cat.
3) The Penguin is a possibility, though don't think Danny Devito stupidity, think more along the lines of Raymond Calitri from "Gone in 60 Seconds" only less of a doufus, sort of a Daniel Craig meets Jason Statham. A British guy, maybe a hit man/import mob boss, wears a Tuxedo, that sort of thing. Maybe even has an umbrella that's also a gun, and many other concealable type weapons.
4) League of Shadows (Assassins). Sure, Ra's died in the first batman, though why couldn't someone else take over the League of Shadows (or another Ra's, which is just a symbol, like Batman himself)? Say, his daughter? Limitless possibilities here, including the fact that the Ra's we knew in the movie survived (unlikely, but possible). Also there's literature that suggests Ra's actually was the head of two factions, perhaps the combination of League of Assassins and Society of Shadows (into League of Shadows) was intentional in the movie. Maybe the two factions split after Ra's death. Or two new emerged.
5) Definitely no Poison Ivy or Mr. Freeze. And ABSOLUTELY no Robin.
6) The Riddler is definitely a possibility. Though again, no funky green suit. More of a serial killer with a knack for leaving riddles. Think Joker, but with more hint dropping and less theatrics. And his name is definitely not Edward Nygma. Just some guy targeting the Gotham elite. Again, could be a secondary villain to Two Face.
7) The Joker. As long as you can find someone to pull it off, there was enough going on in makeup so that the look wouldn't be too far off. But man, that was some Oscar winning stuff Ledger pulled off, so good luck with that.
8) Man-bat I find sort of dumb, unless there's some guy out there copying batman, trying to bring down Batman's name. Of course, you couldn't call him 'Man-Bat' in the movie and get a serious reaction, nor could he be half bat and half man. He'd have to be some sort of diabolical copy-cat.
9) Bane is possible. Though I would think more along the lines of an Insane Arkham prisoner. And no silly purple outfit, or S&M mask.
